A metal-air battery is a rechargeable battery using metal (such as lithium) as the negative electrode active material and oxygen as the positive electrode active material. The positive electrode active material, oxygen, can be obtained from the air, so that it is not needed to encapsulate the positive electrode active material in the battery. In theory, therefore, the metal-air battery can provide a larger capacity than a secondary battery using a solid positive electrode active material.
In the metal-air battery, the reaction described by the following formula (1) proceeds at the negative electrode upon discharging:2Li→2Li++2e−  (1)
Electrons generated by the formula (I) pass through an external circuit, work by an external load, and then reach the positive electrode. Lithium ions (Li+) generated by the formula (I) are transferred by electro-osmosis from the negative electrode side to the positive electrode side through an electrolyte sandwiched between the negative and positive electrodes.
Upon discharging the battery, the reactions described by the following formulae (2) and (3) proceed at the positive electrode:2Li++O2+2e−→Li2O2  (2)2Li++1/2O2+2e−→Li2O  (3)
The thus-produced lithium peroxide (Li2O2) and lithium oxide (Li2O) are stored in the air electrode as a solid.
Upon charging the battery, a reaction which is reverse to the reaction described by the formula (1) proceeds at the negative electrode, while reactions which are reverse to the reactions described by the formulae (2) and (3) proceed at the positive electrodes. Lithium metal is thus regenerated at the positive electrode. Because of this, discharge becomes possible again.
In conventional metal-air batteries, a solid product comprising lithium peroxide (Li2O2) and lithium oxide (Li2O), which are reaction products of the above formulae (2) and (3), is deposited in the air electrode. As a result, the air electrode is clogged up with the solid product to block the contact between the air and a liquid electrolyte, resulting in a problem with charge and discharge.
As a lithium-air battery technique for solving such a solid deposition in the air electrode, Non-Patent Literature 1 discloses a technique for preventing lithium oxide (Li2O), which is a solid reaction product in the air electrode, from deposition by providing an organic liquid electrolyte between the negative electrode and a lithium ion-conducting solid electrolyte, and an aqueous liquid electrolyte between the air electrode and the solid electrolyte.